This invention relates generally to optical discs used for data storage and more specifically to drive electronics used in conjunction with an optical head for detecting data and position information.
Drives for optical discs, for example for Compact Discs (CD) and Digital Versatile Discs (DVD), must control the radial position of an optical transducer head in order to follow a track on a disc, must control the distance of the head from a data surface in the disc (focus control), must extract data from a read signal, and may also detect disc tilt. Drives that can write data onto an optical disc must also extract a clock signal used during writing.
For rewritable optical discs, some formats use embossed lands and grooves in the data layer, with the side walls of the grooves following a generally sinusoidal shape. The resulting grooves are called xe2x80x9cwobbledxe2x80x9d grooves. The wobbled grooves may be used to generate a signal that provides a frequency and phase reference for a clock signal during writing. The frequency of the wobble may be an integer submultiple of the frequency of the write clock signal, or the frequency of the wobble may be higher than the write clock frequency. The walls of the grooves may also be embossed with perturbations that can be used to encode information, such as address information, for example, sector number or block number. In this patent document, the data written by a laser within a groove is called primary data and data that may be embossed onto the wall of a groove is called wobble data.
For one example DVD format, the minimal length of a data mark followed by the minimal length of a gap between marks corresponds to a time period, in the data signal, equal to the time periods of six clock cycles of the read clock. As a result, the signal resulting from reading data marks has a dominant frequency component that is one sixth the frequency of the read clock. In this patent document, the term xe2x80x9cprimary data frequencyxe2x80x9d means the dominant frequency in the data signal, not the read clock frequency.
In many optical disc drives, an optical detector has four sections (called a quad detector) that generate four separate signals, commonly called A, B, C, and D. The primary data signal is the sum of the four quad detector signals (A+B+C+D). A radial position error signal, called a Radial Push-Pull (RPP) signal is derived by subtracting appropriate pairs of the quad detector signals, for example (A+D)xe2x88x92(B+C). For media with wobbled grooves, the wobble signal is a high frequency modulation of the relatively low frequency RPP signal. Each of these various signals (primary data signal, radial position error signal, focus error signal, and wobble signal) requires corresponding electronics to extract, decode, and condition one particular signal from a combination of signals from the quad detector.
FIG. 1 (prior art) is a simplified block diagram illustrating typical drive electronics, with a separate channel for each signal. For reading primary data and generating a read clock, an automatic gain control (AGC) amplifier 100 is followed by an appropriate filter 102, followed by a zero crossing detector 103, then a phase-locked-loop (PLL) 106, and a data detector 108. For reading wobble data and generating a write clock, an amplifier 110 is followed by an appropriate filter 112, followed by a zero crossing detector 114, then a phase-locked-loop (PLL) 116, and a data detector 118. From FIG. 1, it can be seen that the channel for reading primary data, and the channel for reading wobble data, have functionally similar blocks. However, specific design parameters are different between the two channels. In order for written data patterns to be precisely aligned relative to the physical wobble patterns, the wobble channel requires minimal phase delay and nonvarying phase delay. In contrast, for the primary data channel, phase delay is of relatively little concern, and the primary design parameter is low noise at the primary data frequency. AGC may introduce variable phase delay, and therefore cannot be used in amplifier 110. Filter 112 is optimized for low-phase-delay, whereas filter 102 is optimized to boost the primary data frequency and to suppress as much noise as possible. Finally, if the wobble frequency is different than the primary data frequency (which is the usual case), then PLL 116 must be designed to run at a different frequency than PLL 106 (that is, the inputs to the PLL""s have different frequencies).
Also illustrated in FIG. 1 are channels for a tracking error signal (RPP) and focus error signal. For the tracking error signal, an amplifier 120 and a simple low-pass filter 122 are sufficient. Likewise, for the focus error signal, an amplifier 124 and a simple low-pass filter 126 are sufficient. In general, the tracking error signal and the focus error signal have much lower bandwidths than the wobble clock or the data signal.
There is a general need for cost reduction of the drive electronics.
The read channel electronics and the wobble channel electronics are combined into one common channel for reduced cost. When the drive is reading, the common channel is used for primary data and a read clock. When the drive is writing, the common channel is used for wobble data and a write clock. Two example embodiments are provided. In each example, a single front end amplifier, without automatic gain control, is used. In each example, a single PLL and a single data detector are used.